Tackling your sleep, eating five different coloured fresh vegetables and two pieces of different kinds of fruit a day will help amazingly. Check out all of your local universities to see if anyone needs participants in sleep trials. There might even be something online. That kind of formal structure can help nudge you in the right direction. Poor sleep is a curse.
Could be that you are a perfectionist who is procrastinating.
This was a significant hurdle for me in the past, and sometimes it still creeps up on me. I would reread children's books rather than complete assessment items. If I did attempt the assignments I would leave it sooo long, often past the due date, that there wasn't time for me to be 'perfect' in how I wrote the essay. Not that I would have couched it in those terms or recognised what I was doing.
Must admit this tortured me for years, to the point of my being excluded from universities and courses.
Saw a psychiatrist, recognised what was going on, got into cognitive therapy, and found my way through.
Can't recommend good counselling enough. If you are somewhere that doesn't offer affordable health care, or if your school doesn't have a counsellor, check out if any of your local universities are doing any kind of studies in the educational or psychological faculties that would be of use to you. Ask to participate.
Meet with each of your teachers and explain you are having real problems studying, but that you are trying to find solutions. Ask for their advice. Every single teacher. At the very least this will flag for them that you are struggling and trying, and at best they will offer advice that helps.
Visit your local library and get yourself some books on cognitive therapy. Try and work out what it is that is scaring you about failing. My fear of failing was so big it was making me fail! Ha!
By not completing assignments or giving myself enough time to complete assignments my poor grades could never be attributed to my not being smart enough or talented enough, or gifted enough to succeed.
As an adult I can now grasp that it is a-okay to not be perfect at writing. Witness this reddit post. Being good at writing is my Holy Grail but I have to really work to get past the perfectionist craziness, as that stops me from feeling able to try.
This may be miles away from your experience. Hang in there and keep looking inside yourself and externally. Hope you find insight into why this is happening soon, and develop useful strategies that work for you. Good luck!